A conventional electric tool switch mechanism includes a power-source contact switch connected in series with a DC motor and a battery power source, a trigger switch for determining a rotational speed of the DC motor depending on a pressed amount, a semiconductor switching device connected in series with the DC motor and the battery power source via the power-source contact switch, a second contact switch connected in parallel with the semiconductor switching device, and a control circuit for receiving a power voltage via the power-source contact switch and controlling on/off of the semiconductor switching device (see Japanese Patent Laid-open Application No. Hei 6-14576, pages 4 and 5 and FIG. 3).
In the electric tool switch mechanism, when the trigger switch is slightly pressed, the power-source contact switch is on and an operation voltage is supplied to the control circuit. In this case, the control circuit increases or decreases an on-duty of the semiconductor switching device depending on the pressed amount of a trigger switch in order to rotate the DC motor at a speed dependent on the pressed amount. When the trigger switch is pressed at its maximum, the second contact switch becomes on and bypasses the semiconductor switching device so that the power voltage is directly supplied to the DC motor. This prevents any possible loss incurred by internal resistance of the semiconductor switching device.
When the trigger switch is released to stop the rotation of the DC motor and the power-source contact switch is changed from on to off in the state where the semiconductor switching device remains powered on, an arc is generated at the power-source contact switch and the lifetime of the switch contact is reduced. Accordingly, in the conventional electric tool switch mechanism, when the pressed amount of the trigger switch is below a predetermined reference value, the on-duty of the semiconductor switching device is made 0% before the power-source contact switch is changed from on to off, so that the power-source contact switch is changed from on to off in the state where the semiconductor switching device is off.
When the electric tool switch mechanism is continuously used in the state where the trigger switch is fully pressed, the battery is over-discharged. Accordingly, a voltage detecting circuit for detecting the battery voltage is provided, and a central processing unit (CPU) in the control circuit forcibly turns the switching device off when the battery voltage detected by the voltage detecting circuit is below the predetermined reference value, in order to prevent over-discharge of the battery. However, as a user fully presses the trigger switch, the second contact switch connected in parallel with the semiconductor switching device is closed such that discharge current flows through the second contact switch, thereby over-discharging the battery.
Furthermore, in the case where the control circuit increases or decreases the on-duty of the semiconductor switching device depending on the pressed amount of a trigger switch, the on-duty of the semiconductor switching device is obtained by converting the pressed amount of the trigger switch into a voltage value, averaging it, removing noise from the voltage value, and performing operation on the voltage value. This increases an operation time in the CPU of the control circuit. Accordingly, even though the power-source contact switch is changed from on to off as the trigger switch is released, the on-duty of the semiconductor switching device may not be made 0% due to voltage measurement or operation delay. If the semiconductor switching device remains turned on when the power-source contact switch is changed from on to off, the switch contact opens in the state where current is flowing through the power-source contact switch, thereby generating an arc and reducing the lifetime of the contact.